The present invention relates to subterranean well cementing operations, and more particularly, to cement compositions having improved mechanical properties and associated methods.
Hydraulic cement compositions are commonly utilized in subterranean well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby strings of pipe such as casing and liners are cemented in well bores. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between the walls of a well bore and the exterior surface of the pipe string disposed therein. The cement composition is permitted to set in the annular space, thereby forming an annular sheath of hardened substantially impermeable cement therein that substantially supports and positions the pipe string in the well bore and bonds the exterior surfaces of the pipe string to the walls of the well bore. Hydraulic cement compositions also are used in remedial cementing operations such as plugging highly permeable zones or fractures in well bores, plugging cracks in holes in pipe strings, and the like.
Set cement in wells, and particularly the set cement sheath in the annulus of a well, may fail due to, inter alia, shear and compressional stresses exerted on the set cement. This may be particularly problematic in high temperature wells, which are wells wherein fluids injected into the wells, or produced from the wells by way of the well bore, causes a temperature increase over initial cement setting conditions of at least about 100° F. In these types of wells, set cements often fail as a result of the stresses exerted on the set cement.
The stress exerted on the cement as referred to herein means the force applied over an area resulting from the strain caused by the incremental change of a body's length or volume. The stress is generally thought to be related to strain by a proportionality constant known as Young's Modulus. Young's Modulus is known to characterize the flexibility of a material. In a well bore sealing application, the Young's Modulus for nonfoamed cements is about 3×106 lbf/in2, and for steel casings, the Young's Modulus is about 30×106 lbf/in2.
There are several stressful conditions that have been associated with well bore cement failures. One example of such a condition results from the relatively high fluid pressures and/or temperatures inside of the set casing during testing, perforation, fluid injection, or fluid production. If the pressure and/or temperature inside the pipe increases, the resultant internal pressure expands the pipe, both radially and longitudinally. This expansion places stress on the cement surrounding the casing causing it to crack, or the bond between the outside surface of the pipe and the cement sheath to fail in the form of, inter alia, loss of hydraulic seal. Another example of such a stressful condition is where the fluids trapped in a cement sheath thermally expand causing high pressures within the sheath itself. This condition often occurs as a result of high temperature differentials created during production or injection of high temperature fluids through the well bore, e.g., wells subjected to steam recovery processes or the production of hot formation fluids. Other stressful conditions that can lead to cement failures include the forces exerted by shifts in the subterranean formations surrounding the well bore or other over-burdened pressures.
Failure of cement within the well bore can result in radial or circumferential cracking of the cement as well as a breakdown of the bonds between the cement and the pipe or between the cement sheath and the surrounding subterranean formations. Such failures can result in at least lost production, environmental pollution, hazardous rig operations, and/or hazardous production operations. A common result is the undesirable presence of pressure at the well head in the form of trapped gas between casing strings. Additionally, cement failures can be particularly problematic in multi-lateral wells, which include vertical or deviated (including horizontal) principal well bores having one or more ancillary, laterally extending well bores connected thereto.
In both conventional single bore wells and multi-lateral wells having several bores, the cement composition utilized for cementing casing or liners in the well bores must develop high bond strength after setting and also have sufficient resiliency, e.g., elasticity and ductility, to resist loss of pipe or formation bonding, cracking and/or shattering as a result of all of the stressful conditions that may plague the well, including impacts and/or shocks generated by drilling and other well operations.